GSM - Architecture, Protocols and Services, Third Edition

Chapter 5. Protocols

Protocol architecture planes

The various physical aspects of radio transmission across the GSM air interface and the realization of physical and logical channels were explained in Chapter 4. According to the terminology of the OSI Reference Model, these logical channels are at the Service Access Point of Layer 1 (physical layer), where they are visible to the upper layers as transmission channels of the physical layer. The physical layer also includes the forward error correction and the encryption of user data.

The separation of logical channels into the two categories of control channels (signaling channels) and traffic channels (Table 4.1) corresponds to the distinction made in the ISDN Reference Model between user plane and control plane. Figure 5.1 shows a simplified reference model for the GSM User–Network Interface (UNI) Um, where the layer-transcending management plane is not elaborated on further in the following. In the user plane, protocols of the seven OSI layers are defined for the transport of data from a subscriber or a data terminal. User data are transmitted in GSM across the air interface over traffic channels TCH, which therefore belong to Layer 1 of the user plane (Figure 5.1).

Protocols in the signaling plane are used to handle subscriber access to the network and for the control of the user plane (reservation, activation, routing, switching of channels and connections). In addition, signaling protocols between network nodes are needed (network ...